The present invention pertains to displays and, particularly, to liquid crystal displays (LCD's). More particularly, the invention pertains to active matrix LCD's.
LCD technology is being developed as a possible successor to cathode ray tube (CRT) technology for many applications. LCD technology offers important advantages, such as higher reliability and reduced power, size and weight. However, in the current state of development, LCD image rendering capability falls short of that achievable using CRT's. The present invention addresses one of the major technical obstacles which is the unacceptable flicker of flat panel LCD's.
The flicker problem originates in the manner that LCD's are driven. Flat panel LCD's need to be refreshed periodically with alternating voltage. The polarity of the voltage is typically switched after each vertical sync in order to prevent electroplating action from occurring. Electroplating action can damage electrodes inside the flat panel. Odd frames 11 of the image are driven by a minus voltage (-V), for example, while even frames 12 are driven by a positive voltage (+V) (in FIGS. 1a and 1b). Since the electro-optical response of the LC material depends solely on the magnitude of the voltage (V), polarity changes after each frame should have no optical effect. However, polarity changes do have a noticeable effect.
FIGS. 1a and 1b reveal the prior art off-axis output of an LCD. FIG. 1a is a graph showing the idealized average level of optical output per frame, 11 or 12, relative to the voltage polarities of the driving signals, as distributed temporally. FIG. 1b shows how the polarity dependent regions are distributed spatially over a display surface. The regions are switched to the opposite polarity at the end of each frame. Each of the regions cover the entire image display. The optical output has a frequency component that is one-half of the frame frequency.
Active matrix LCD technology is preferred in cockpit applications, because it has great potential for realizing the required level of performance under adverse conditions. Active matrix displays typically use semiconductor devices as switches (most often thin film transistors) to transfer appropriate voltages to each LC picture element (i.e., pixel). Although these switching devices are designed to behave independently of polarity, they exhibit asymmetric properties. They appear to charge faster or conduct better for one polarity than for the other. Consequently, active matrix LC Displays, using such polarity-dependent devices to energize the LC material, manifest polarity-dependent optical behavior (FIGS. 2a and 2b). This polarity-dependent optical behavior is perceived as flicker by the eye. FIGS. 2a and 2b are graphs that reveal the output of an LCD having switched polarities, at a plus 45 degree viewing angle and a minus 45 degree viewing angle, respectively.
Part of the flicker effect can be tuned out for a given viewing angle by adjusting the magnitudes of the applied voltages. Some display designers in the industry have found this to be an adequate solution. The voltages are adjusted to compensate for the polarity dependence. For example, the magnitude of +V may be made slightly higher than that of -V to account for biases in the active matrix LCD. However, because of the complex characteristics of LCD's, such tuning fails when the panel is viewed from other angles. So, for applications requiring wide viewing angles this solution is inadequate.
In general, if the panel is refreshed at frequency (F), then the polarity must be alternated at every half period or at frequency F/2. Because of the asymmetries mentioned above, polarity alternation causes the optical output of the LC display to have an undesirable side effect; the image gets modulated at F/2. An image refreshed at 60 hertz (Hz) will cause a 30 Hz frequency component to appear over the entire surface of the screen. 30 Hz results in very perceptible and objectionable flicker. On this basis, those skilled in the art conclude the refresh frequency must be raised to the point where F/2 is high enough to avoid flicker. In the inventors' laboratory, the refresh frequency had to be raised to 90 Hz. However, high refresh frequencies have severe penalties associated with them. Such frequencies raise the complexity, speed and cost of the entire display system. Transistors in the LCD must be designed to operate faster. The graphics processors, image memories and interface circuitry in the symbol generator and the display head require higher performance components and must use more costly architectures which are items to be avoided whenever possible.